Metal forming processes may be described as operations in which metal undergoes a plastic deformation to obtain the desired shape of the workpiece. Typically, no metal is removed in a forming process. Examples of metal forming processes include hot and cold rolling, forging (of both ferrous and non-ferrous metals), molding, stamping, casting, ironing, drawing, and extruding. Metal forming processes differ from metal working processes. In metal working processes metal is removed to obtain the desired shape of the workpiece. Examples of metal working processes include cutting, drilling, turning, and milling.
In all the metal forming processes it is necessary to lubricate the surface of the metal and of the tools with which it is being formed. The most commonly used lubricant compositions comprise colloidal suspensions of fine particulate graphite in oil or water. These suspensions are often inconvenient to handle and to deliver to the working surfaces, constitute a health hazard for the operator, and tend to lead to the formation of graphite deposits on the metal or on the tool.
Many other lubricants have been proposed for use in metal forming operations. These include semi-synthetic or synthetic emulsions containing inorganic salts, fatty acid metal soaps, and organic esters.
Klaus, U.S. Pat. No. 3,978,908, describes the introduction of a vaporized lubricant composition to the surface of a die or mold. One preferred class of lubricants described in this patent are the triaryl phosphates, especially tricresyl phosphate.
Graham, U.S. Pat. No. 5,584,201, incorporated herein by reference, describes the use of a water based lubricant composition comprising a tri(alkylaryl)phenyl phosphate and a die release agent to lubricate the surface of a metal forming die. The die release agent is either a binder, such as a. lignosulfonate, a water soluble cellulose compound, or a fatty acid soap. The preferred phosphate ester is a butylated phenyl phosphate ester.
Metal forming lubricant compositions are typically sold as concentrates. These concentrates are emulsions, which are diluted by the user prior to use. The concentrates are prepared by the manufacturer and shipped in drums to the user, who may store the drums of concentrate for several weeks to months prior to use. Because the lubricant properties of the metal forming lubricant composition are typically lost if the lubricant deemulisifies, the emulsion should have a shelf life (stability) at room temperature (about 25.degree. C.) of at least one month, preferably at least six months, and more preferably at least one year. A high temperature (about 75.degree. C.) shelf life of at least one month is preferred.
The water based emulsion concentrates and emulsions described in Graham are unstable. These emulsion concentrates comprise significant quantities of the sodium salt of a fatty acid and separate on standing. This instability is further exacerbated by the presence of inorganic extreme pressure agents. This deterioration diminishes its lubricating properties, requiring the emulsion be disposed of frequently. Thus, a need exists for lubricant compositions for metal forming that exhibit good stability upon prolonged storage and also offer significant advantages in performance.